This invention relates to a circuit arrangement for switching a current in a load which includes an inductive reactance, comprising at least one transistor which switches through its collector-emitter path the current in the load and whose base is connected to the emitter through a voltage-stabilizing element for stabilizing the value of a voltage supplied to the base, as well as a dissipation device for dissipating currents or voltages occurring in a sense opposite to the sense of conduction of the transistor and produced by the inductive reactance.
From the "International Rectifier HEXFET Data Book" HDB-2, Chapter 12, Application Note 946, p. A-111 to A-122, especially FIG. 11, of 1982, such a circuit arrangement is known and comprises an electronic switch (e.g. a bipolar transistor) by means of which the current is switched in a load which comprises a series arrangement of an inductor and a capacitor. The current in the load then flows through the collector-emitter path of the bipolar transistor, having a base supplied with a switching voltage which is produced in a driver circuit. In order to stabilize the value of this switching voltage in the cutoff condition of the bipolar transistor, the base is coupled via a Zener diode to the emitter of the bipolar transistor.
In the known circuit arrangement, the main current path of a field effect transistor is also included in the path of the current through the load, i.e. in series with the collector-emitter path of the bipolar transistor. The gate electrode of this field effect transistor is supplied by a control circuit with a control voltage for controlling the current in the load. The control circuit then supplies only the very small gate current of the field effect transistor. Thus, it becomes possible for the electronic switch to be directly driven by logic switching circuits.
When the current in an inductive load is switched off, voltage peaks or overshoot pulses are obtained because of the self-inductance. These overshoot pulses have a sense opposite to that of the current flowing through the electronic switch in the conducting condition or of the voltage applied thereto in its cut-off condition after switching transients have decayed. These voltage peaks may lead very rapidly to a destruction of the transistor. Therefore, they are dissipated through a dissipation device, preferably a fly-back diode, which is arranged parallel to the series arrangement of the collector-emitter path of the bipolar transistor and the main current path of the field effect transistor and with such a polarity that it remains cut off for the voltage applied in the cut-off condition of the transistor between its collector and its emitter after switching transients have decayed. In circuit arrangements for switching a high voltage at a load having a high inductive reactance, this diode must have on the one hand a high cut-off voltage and must be designed on the other hand for a high peak current. Such diodes are comparatively expensive.